1. Field of the Invention
The present invention relates to a vibratory flow meter and method, and more particularly, to a vibratory flow meter and method for correcting for entrained gas in a flow material.
2. Statement of the Problem
Vibrating conduit sensors, such as Coriolis mass flow meters and vibrating densitometers, typically operate by detecting motion of a vibrating conduit that contains a flowing material. Properties associated with the material in the conduit, such as mass flow, density and the like, can be determined by processing measurement signals received from motion transducers associated with the conduit. The vibration modes of the vibrating material-filled system generally are affected by the combined mass, stiffness and damping characteristics of the containing conduit and the material contained therein.
A typical Coriolis mass flow meter includes one or more conduits that are connected inline in a pipeline or other transport system and convey material, e.g., fluids, slurries and the like, in the system. Each conduit may be viewed as having a set of natural vibration modes, including for example, simple bending, torsional, radial, and coupled modes. In a typical Coriolis mass flow measurement application, a conduit is excited in one or more vibration modes as a material flows through the conduit, and motion of the conduit is measured at points spaced along the conduit. Excitation is typically provided by an actuator, e.g., an electromechanical device, such as a voice coil-type driver, that perturbs the conduit in a periodic fashion. Mass flow rate may be determined by measuring time delay or phase differences between motions at the transducer locations. Two such transducers (or pickoff sensors) are typically employed in order to measure a vibrational response of the flow conduit or conduits, and are typically located at positions upstream and downstream of the actuator. The two pickoff sensors are connected to electronic instrumentation by cabling, such as by two independent pairs of wires. The instrumentation receives signals from the two pickoff sensors and processes the signals in order to derive a mass flow rate measurement.
Flow meters are used to perform mass flow rate measurements for a wide variety of fluid flows. One area in which Coriolis flow meters can potentially be used is in the metering of oil and gas wells. The product of such wells can comprise a multiphase flow, including the oil or gas, but also including other components, including water and air, for example. It is highly desirable that the resulting metering be as accurate as possible, even for such multiphase flows.
Coriolis meters offer high accuracy for single phase flows. However, when a Coriolis flow meter is used to measure aerated fluids or fluids including entrained gas, the accuracy of the meter can be significantly degraded. Entrained gas is commonly present as bubbles in the flow material. The size of the bubbles can vary, depending on the amount of gas present, the pressure of the flow material, the temperature, and the degree of mixing in the pipeline. The extent of the decrease in performance is not only related to how much total gas is present, but also to the size of the individual gas bubbles in the flow. The size of the bubbles affects the accuracy of the measurement. Larger bubbles occupy more volume, leading to fluctuations in the density of the flow material. Due to the compressibility of a gas, the bubbles can change in gas amount yet not necessarily change in size. Conversely, if the pressure changes, the bubble size can correspondingly change, expanding as the pressure drops or shrinking as the pressure increases. This can also cause variations in the natural or resonant frequency of the flow meter.
Another problem caused by gas bubbles is slippage. Small bubbles typically move with the liquid flow material as the flow meter is vibrated. However, larger bubbles do not move with the liquid during vibration of the flow tube. Instead, the bubbles can be decoupled from the liquid and can move independently of the liquid. Consequently, the liquid can flow around the bubbles. This adversely affects the vibrational response of the flowmeter.
There remains a need in the art for a vibratory flow meter that detects problematic levels of entrained gas. There remains a need in the art for a vibratory flow meter that can accurately measure flow characteristics in the presence of entrained gas. There remains a need in the art for a vibratory flow meter that can accurately measure flow characteristics at any level of entrained gas.